1. Field of the Invention
Embodiments of the invention generally relate to a cleaning solution for use during the fabrication of semiconductor devices. More particularly, the invention relates to cleaning solutions and methods for using the cleaning solutions prior to depositing metal alloy capping layers over conductive layers in electronic devices.
2. Description of the Related Art
Recent improvements in circuitry of ultra-large scale integration (ULSI) on substrates indicate that future generations of semiconductor devices will require multi-level metallization with smaller geometric features. The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio features, including contacts, vias, lines and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual substrates and die as features decrease below 0.13 μm in size.
Currently, copper and its alloys have become the metals of choice for sub-micron interconnect technology because copper has a lower resistivity than aluminum, (1.67 μΩ-cm compared to 3.1 μΩ-cm for aluminum), a higher current carrying capacity, and significantly higher electromigration resistance. However, copper diffuses through substrate and may still electromigrate, thereby requiring a barrier layer between the copper and the substrate. Also, copper readily forms copper oxide when exposed to atmospheric conditions such as environments outside of processing equipment. Copper oxide formation increases the resistance of metal layers (e.g., copper interconnects) and reduces the reliability of the overall circuit.
Selectively depositing a capping layer of a metal alloy on the copper provides an efficient barrier to copper diffusion, electromigration and oxidation. This appears most readily accomplished using an electroless plating process selective for copper relative to a dielectric material of the substrate. In this manner, the capping layers that may be made from cobalt alloys such as cobalt tungsten boron (CoWB) or cobalt tungsten phosphorous (CoWP) may be deposited from a plating solution that may either be self initiated or activated by displacement plating, such as with palladium or any other relevant catalyst. However, copper oxide detrimentally affects electroless deposition which requires a surface capable of electron transfer for nucleation since oxidized surfaces cannot participate in proper electron transfer and copper oxide may contain contaminants within which interfere with alloy deposition. Further, the dielectric material may have contaminants thereon that the capping layer deposits to during the electroless plating process. Deposits of the capping layer on the dielectric material and extensions of the capping layer grown at the edges of the copper lines can cause shorts in the electrical circuit. Contaminants on the copper and the dielectric material include oxides, copper oxides, copper-organic complexes, silicon oxides, benzotraixole (BTA), TTA, resist, polymeric residue, derivatives thereof and combinations thereof. Thus, the substrate is cleaned of various contaminants prior to depositing the capping layer in order to selectively and efficiently deposit the capping layer on the copper. The cleaning of the substrate is typically done after polishing or leveling the copper or other conductive material, such as by chemical mechanical polishing (CMP), and prior to deposition of the capping layer. The cleaning, the optional activating, and the depositing of the capping layer may be performed on the substrate in multiple steps.
Various cleaning solutions that are known and used to clean copper and dielectric materials at other stages during the manufacture of the semiconductor have disadvantages when used prior to depositing the capping layer. Examples of known cleaning solutions for exposing to the substrate in order to clean the dielectric material or the copper include acidic solutions (e.g. hydrofluoric acid (HF), citric acid, amino acids such as glycine, sulfuric acid (H2SO4) and/or hydrochloric acid (HCl)) and Electra Clean™ solutions commercially available from Applied Materials, Inc., of Santa Clara, Calif. However, the prior cleaning solutions can corrode the copper, etch the copper, roughen the copper surface, and damage the dielectric material. Additionally, the copper grain boundaries can be decorated by the prior cleaning solutions thereby detrimentally decorating the geometric grain or crystal structure of the copper. Further, the prior cleaning solutions can go between the barrier layer and the copper or between the barrier layer and the dielectric material and damage these boundaries.
Therefore, there exists a need for cleaning solutions and methods for using the cleaning solutions that can simultaneously clean conductive layers and dielectric materials prior to depositing metal alloy capping layers over the conductive layers. There exists a further need for cleaning solutions and methods that more effectively remove contaminants without substantially adversely affecting the interconnect formed therefrom.